Optical angular measurement sensors

ABSTRACT

Systems that enable observing celestial bodies during daylight or in under cloudy conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. application Ser.No. 16/025,185, filed Jul. 2, 2018, entitled OPTICAL ANGULAR MEASUREMENTSENSORS, which is a continuation of U.S. application Ser. No.15/632,767, filed Jun. 26, 2017, now U.S. Pat. No. 10,012,547, which isa continuation of U.S. application Ser. No. 15/155,939, filed May 16,2016, entitled OPTICAL ANGULAR MEASUREMENT SENSORS, now U.S. Pat. No.9,689,747, which in turn is a continuation of U.S. application Ser. No.14/216,459, filed Mar. 17, 2014, now U.S. Pat. No. 9,341,517, which inturn claims priority to and benefit of U.S. Provisional Application No.61/799,699, filed Mar. 15, 2013, the entire contents of which areincorporated herein by reference in their entirety and for all purposes.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made partially with U.S. Government support from theU.S. Marine Corps under Contract No. M67854-09-C-6511. The U.S.Government has certain rights in the invention.

BACKGROUND

These teachings relate generally devices for observing celestial bodiesduring daylight or nighttime.

There is a need for systems that enable observing celestial bodiesduring daylight or under cloudy conditions.

SUMMARY

The various embodiments of the present teachings disclose systems thatenable observing celestial bodies during daylight or in under cloudyconditions.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the accompanyingdrawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the Spectral Sensitivities of Focal Plane Arrays Made FromSilicon and Three Forms of InGaAs;

FIG. 2 shows Relative Power Density as a Function of Wavelength for aSun Temperature Black Body outside the Atmosphere;

FIG. 3 shows representation of a color image;

FIG. 4 shows an Airglow Spectrum;

FIG. 5 a shows a Photograph of Venus (0.85-1.0 micron IR) Before Dawn;

FIG. 5 b shows a Photograph of Venus (0.85-1.0 micron IR) During Dawn;

FIG. 5 c shows a Photograph of Venus (0.85-1.0 micron IR) Near Sunrise;

FIG. 5 d shows a Photograph of Venus (0.85-1.0 micron IR) Well AfterSunrise;

FIG. 6 shows Sequential Daylight Images of Venus in 1.3-1.7 micron IRBand;

FIG. 7 shows a Daylight Image of Venus in 0.9-1.0 micron IR Band;

FIG. 8 shows a Detail of Daylight Image of Venus in 0.9-1.0 Micron IRBand;

FIG. 9 shows a Near IR Daylight Image of Betelgeuse;

FIGS. 10-13 show daylight images of various stars obtained using thesystem of these teachings;

FIG. 14 shows Experimentally Measured SNRs for Daytime Star Imagesobtained using the system of these teachings;

FIG. 15 shows an Image from a wide angle imager obtained using thesystem of these teachings;

FIG. 16 illustrates the earth (center) surrounded by a Celestial Sphereas used in the system of these teachings;

FIG. 17 shows an LWIR Image of Sun in Light Clouds obtained using thesystem of these teachings;

FIG. 18 shows Visible (TOP) and LWIR (BOTTOM) photographs of the Sky,the LWIR image obtained using the system of these teachings;

FIG. 19 shows Visible (Top) and LWIR (Bottom) Photographs of the Sun,the LWIR image obtained using the system of these teachings;

FIG. 20 shows one view of an embodiment of the system of theseteachings;

FIG. 21 shows visible (top two) and LWIR (Bottom) Photographs, the LWIRimage obtained using the system of these teachings; and

FIG. 22 shows a block diagram representation of one embodiment of thesystem of these teachings.

DETAILED DESCRIPTION

This invention relates in part to devices for observing celestial bodiesduring daylight or nighttime. This function allows a measure of angles,such as azimuth, from identifying the positions of these stars orbodies. One term for labeling many of the embodiments of theseinventions is Optical Azimuth Sensor, or OAS.

In order to understand the expected operation of the optical azimuthsensor during bright sunlight conditions it is necessary to consider theoptics of the sky. In planets (or the moon) without an atmosphere, thesky is black both at nighttime and during daylight. On such a planet,conventional celestial navigation and the optical azimuth sensor wouldclearly function well both at night and during the day. On Earthhowever, the atmosphere scatters sunlight, and as a result duringdaytime the sky is bright and blue. Other than in clouds (where thescatterers are relatively large droplets of water), many of thescatterers in the atmosphere are very small compared to the wavelengthof light and the resulting scatter is described very accurately as aRaleigh Scattering phenomenon. In Raleigh scattering, the amount oflight scattered increases very rapidly as the wavelength of the incidentlight decreases, i.e., blue light is scattered much more strongly thanred light. This is why the sky appears blue.

Quantitatively, in the Raleigh Scattering regime light is scattered inproportion to the inverse of the fourth power of the wavelength. Visiblelight covers a spectral band from 0.4 μm (blue) to 0.7 μm (red). Redlight at that wavelength is scattered by a factor of 9.37 or nearly 10times less than blue light of that wavelength. In other words, if aninstrument were to look at the sky through a blue filter that passed 0.4μm wavelength light, and then through a red filter that passed 0.7 μmwavelength light, the sky would look roughly 10 times brighter throughthe blue filter than through the red filter.

Standard silicon CCD's or silicon focal plane arrays are sensitive towavelengths out to about 1 μm. This is illustrated by spectralsensitivity curve 12 in FIG. 1 . Light with wavelengths of 1 μm isscattered 40 times less than blue light.

Standard InGaAs focal plane arrays are sensitive to wavelengths up to1.7 μm, illustrated in spectral sensitivity curve 14 of FIG. 1 , whichare scattered 326 times less than blue light. Modified InGaAs focalplane arrays are sensitive to wavelengths out to 2.5 μm, shown byspectral sensitivity curves 16 and 18 of FIG. 1 , and these wavelengthsare scattered roughly more than 1525 times less than blue light.

Thus by imaging the sky using a focal plane array through a filter inthe infrared at a band of wavelengths near 1.7 μm, the sky will beroughly 326 times darker than imaging the sky in the visible using bluelight. In a more extreme example, long wave infrared (LWIR) light near14 μm may be scattered roughly a million times less strongly than bluelight.

The light from stars, however, is imaged directly through the atmosphere(sky) and so the star light is essentially just as bright at a givenwavelength as it is at nighttime. [The spectral dependency of thestarlight as a function of wavelength roughly follows a black body curvesuch as curve 20 in FIG. 2 for a 6000 degree Kelvin black bodyrepresenting roughly a 6000 degree Kelvin star, and so there is a smallroll-off in intensity in the infrared (e.g., a decrease of roughly afactor of 2 from the peak irradiance at 0.5 μm to the irradiance at 1.0μm), but this is very small in comparison with the decrease in skybrightness in the infrared].

Further understanding of the embodiments and expected OAS daylightoperation comes from comparing the sky brightness in daytime andnighttime. One can see the bright reference stars as the morning skybrightens until nearly sunrise. At sunrise on a clear day, the ambientilluminance is roughly 40 lux. The illuminance from a clear blue sky atmidday is roughly 20,000 lux. In other words, for an observer theilluminance of this sky is roughly 500 times brighter at midday than atsunrise. Therefore if the OAS images the stars under a bright daylightsky at a wavelength band centered at 1.7 μm, the starlight will beweaker by roughly a factor near 3 (due to blackbody roll-off), but thesky will be darker by a factor of 325. If an observer's eye wassensitive to the 1.7 micron infrared wavelength, the stars may beplainly visible against a fairly dark sky.

The situation is even better, though, since using a focal plane array inthe OAS the point images of the stars can be more readily identifiedfrom a significant background by subtracting the uniform background, andso embodiments of the OAS operate effectively using simple silicon CCDsat a wavelength near 1 μm or even shorter. The visibility of the weakerstars is limited in large part by the background noise (e.g., shotnoise) compared to the signal of the faint star.

Anticipated Benefits of the Technical Innovations

There are many significant advantages that arise from the OAS technicalinnovation proposed here, including:

1. Compact Size. The OAS includes imager, focal plane, and FPGA ormicroprocessor in single miniaturized package (e.g., in one embodimenttargeted toward roughly 1 cubic inch).

2. Low Mass. The tiny sensor is expected to weigh only tens of grams.

3. High Accuracy. A 4-megapixel CCD array should provide an accuracy ofroughly 0.06° resolution. Higher and lower angular resolutions arepossible.

4. Minimal Setup Time. The OAS is simply exposed to the sky and digitaldata should be available after a short processing time (e.g., a fractionof a second to seconds).

5. Rugged. The OAS is rugged and durable and no focusing is required.

6. Low Power Consumption. The simple electronics are expected todissipate very little power.

7. Insensitive to Magnetic Interference. The OAS senses azimuth angleoptically and is insensitive to magnetic interference.

8. Insensitive to Electronic Jamming. The OAS senses azimuth angleoptically and is insensitive to electronic jamming. The OAS internalelectronics are easily shielded.

9. Not Dependent on Earth's Magnetic Field. Macroscopic shifts in theEarth's magnetic field, as well as local perturbations in the Earth'smagnetic field have no effect on the OAS.

10. Not Dependent on GPS Signals. The OAS senses azimuth angle opticallyand is not dependent on GPS signals.

11. High Precision Digital Output. The OAS is designed to output digitalazimuth angle in standard formats.

12. Not Dependent on Triangulation. Triangulation setup is not requiredfor OAS operation.

13. Referenced to True North. The OAS is readily configured to referenceto True North.

14. Reduced Sensitivity to Smoke and Haze. The Infrared wavelengths usedhave reduced sensitivity to smoke and haze.

15. Readily Integrated in Man-Portable Targeting Equipment. The tiny OASsensor is readily mechanically interfaced to man-portable targetingequipment. The OAS FPGA or microprocessor flexibly outputs data informats compatible with targeting equipment.

16. Potentially Useful In Battlefield Environments. The OAS can be basedon infrared wavelengths that have reduced sensitivity to battlefieldsmoke and haze. The external window protecting the infrared imager isinsensitive to dirt and scratches and is readily cleanable.

17. Insensitive to Moderate Cloud Cover. Moderate cloud cover, even withthick clouds can in some cases be tolerated during OAS operation sincereference stars or celestial objects are scattered over the entire sky.

Experimental Prototype

The operation of the OAS system was demonstrated by building a simpleOAS system using a compact IR camera and IR bandpass filters. Theperformance and star image contrast ratios was measured at a variety ofcenter wavelengths.

The technical innovation in the novel Optical Azimuth Sensor (OAS)includes that it uses a compact, low power, novel optical system toaccurately measure azimuth by identifying the positions of key referencestars (or other celestial bodies such as planets, the moon, or sun) inbright daylight, hazy, as well as nighttime conditions. The essence ofthis novel OAS azimuth sensor is a miniature wide field spectrallyoptimized imager that images the sky onto a high-performance focal planearray. A polar star (such as the North Star in the northern hemisphere),other key bright reference stars or other bodies can be used toaccurately determine True North. Utilizing techniques developed in thisprogram, full functionality of this optical azimuth sensor (OAS) systemis also expected during bright sunlight conditions through one or moreof a variety of modes, as described below.

Detecting celestial objects in daylight to extend celestial navigationto daylight hours is accomplished using the OAS. As described above, amajor component of background light hiding stars during daylight is dueto Rayleigh scattering that is very strongly weighted spectrally towardshort wavelengths (by the fourth power of inverse wavelength!). Byimaging stars at longer wavelengths (filtering out the shorterwavelength highly scattered light) the weak starlight is seen against amuch darker scatter background, as shown in FIG. 1 .

The principle of operation of the OAS is illustrated in FIG. 3 . The topimage 40 in FIG. 3 represents a color image of a star at dawn. Image 42is the blue component of color image 40. Image 44 is the green componentof color image 40. Image 46 is the red (longer wavelength) component ofcolor image 40. Thus the color image is broken into its blue, green, andred channels in the three component images (bottom). The backgroundscattered light is greatly reduced as the wavelength increases, asevidenced by the background becoming much darker. Wavelength bands inthe red and longer (infrared) spectral regions are being targeted fordaylight tracking of celestial bodies. Further enhancement can beobtained by subtracting the uniform background.

FIG. 3 clearly shows that the sky appears much darker through a greenfilter than through a blue filter, and darker still through a red filterthan through a green filter. The detectability of the star is greatlyimproved at longer wavelengths. Spectral bands for OAS operation in thered visible spectrum, as well as the near IR, Short Wave IR, Mid-WaveIR, Long Wave IR, and Very Long Wave IR are being considered (in thecontext of detector performance in those bands) for reduced scatter andenhanced detectability of the stars. However, as discussed below,Rayleigh scattering is not the only mechanism providing backgroundsignal through which the stars must be detected.

When background scatter is sufficiently reduced, another backgroundcomponent called airglow can be significant, as shown in FIG. 4 wheresolar spectrum 60 is compared to airglow spectrum 64. Airglow is abackground electromagnetic radiation that is emitted in the upperatmosphere. One source of airglow is luminescence caused when cosmicrays impinge on the upper atmosphere. The sun also creates ions throughphotoionization which recombine over time and contribute to airglow.Another contribution to airglow is hydroxyl ions reacting with oxygenand nitrogen in the upper atmosphere. Many other mechanisms contributeto airglow during daytime and nighttime conditions. It is apparent inFIG. 4 that spectral gaps or “windows” exist where there is littleairglow flux (e.g., 675-680 nm.). Positioning daytime starlightobservations in spectral windows devoid of airglow will further enhancethe signal-to-noise of the star images particularly during the day. Inoptimizing the OAS operation, edgepass and bandpass filters can be usedto take advantage of both long wavelengths for low scatter backgroundand airglow spectral windows for low emission background.

Airglow spectral features continue into the infrared. By imaging lightfrom targeted celestial bodies at wavelengths in airglow windows (withweak or no emission) the signal-to-noise in celestial body detection canbe further enhanced. [Figure reproduced with permission from L. Cowleywww.atoptics.co.uk].

Selecting optimum stars for Optical Azimuth Sensor use offers furtherOAS system benefits. One of these is Sirius, visually the brightest starwith an apparent magnitude of −1.47. (An increase of 1 in apparentmagnitude corresponds to roughly a decrease in intensity of a factor of2.5.) In another example, the North Star, Polaris (apparent magnitude2.01), is useful in the northern hemisphere since it is nearly fixed inposition throughout the day and night. Polaris effectively rotates dailyin a small circle only 0.7° from the pole axis, and so defines the TrueNorth azimuth accurately twice a day—and is readily corrected usinglook-up tables stored in the OAS. Other objects are also advantageous invarying OAS scenarios. For example the Red (Super) Giant star Betelgeuse(Magnitude 0.58) has many desirable characteristics. First, its emissionsurface is cooler than most other stars, and so it emits preferentiallyin the longer wavelengths (advantageous for OAS application). At thesame time, although it is not as luminous as many other stars at itsemission surface, it is a supergiant star so there is a huge arearadiating into its unresolved image—producing in effect a large apparentbrightness as seen from Earth. This combination of high luminosity andlong wavelength (red) peak emission can be very useful as an OASreference. Further its direction is nearly perpendicular to the Earth'saxis so its image translates on a focal plane at a relatively high rateas the Earth rotates. This translation across focal plane pixels canalso be useful in discriminating the star from background noise. Thisscenario would lend itself to a SIMD processor which can efficientlyprocess such images from nearest neighbor operations.

Other Celestial Objects. In addition or combination with images ofstars, images of planets, the moon, or the sun also form the basis ofrobust OAS configurations. For example, imaging the planets are similarto the stars, but they can be brighter and in some cases are resolved(have more than one resolution cell across their image). For example,the apparent magnitudes of these objects are the Moon (−12.9), Venus(−4.6), Jupiter (−2.9), Mars (−2.9), Mercury (−1.9), and Saturn (−0.2).The moon and the sun also offer some interesting OAS configurations. TheSun can be imaged at long IR wavelengths including the bands describedearlier through the Very Long Wave IR (VLWIR), some of which passreadily through clouds as the wavelengths approach and become largerthan the water droplets. Such an OAS system could is very useful forovercast situations where some other OAS configurations may be less ornot effective. As with the stars, accurate location of the sun's imageas it tracks can be accurately correlated with direction, and highaccuracy can be obtained with internally stored reference data and anaccurate clock/calendar.

Still other OAS embodiments use other filtering approaches, such asnarrow spectral filters in the Frauenhofer spectrum where the daylightis actually dark in narrow absorption bands in daylight. In thisembodiment, the dark absorption lines (e.g., the Frauenhofer Spectrum)superposed on the solar continuum are excellent places to detectreference stars—which may be frequency shifted (into the dark lines) dueto their motion in space. These features are typically very narrow,however, and narrow spectral filters can enable such embodiments of theOAS.

Reference is now made to FIG. 5 . Images 70, 80, 90, and 100 in FIG. 5are photographs of Venus (apparent magnitude of −4.36) taken in thespectral band from 0.85-1.0 microns under the 4 sky conditions 1) beforedawn, 2) during dawn, 3) near sunrise, and 4) well after sunrise,respectively. These four images contain background components 72, 82,92, and 102 respectively; and Venus images 74, 84, 94, and 104respectively. The sky was dark enough in images 70 and 80 to supportlonger exposures, and the image was heavily exposed and translation ofVenus is visible during the exposures. As the sky brightened nearsunrise, the exposures were shorter and the background from the sky wasmore intense. However, even well after sunrise image 100 the image ofVenus is clearly visible and readily detected even without processingand background subtraction. When image 100 was taken, Venus could not bevisually found in the sky.

It is advantageous for the OAS devices to be able to function in themidst of haze and smoke, and ideally even in the presence of partial orfull cloud cover. As the wavelength of the light imaged grows largerthan the scatterers, the ability to image through the scatterersimproves. While cloud particle size distributions seem to vary widelywith cloud type, one measurement on Stratus clouds indicated that thewater fraction and droplet size vary and fall off toward the cloud topand bottom, with a peak in the 5-6 micron effective radius. Embodimentsof the OAS operating at long IR wavelengths, up to and through the VLWIRdescribed earlier, may be additionally useful for OAS operation throughclouds. In one such OAS embodiment the sun is a key element. The sunemits light roughly according to a blackbody curve, and although itsemission falls off at longer IR wavelengths and the atmosphere andclouds absorb a fraction of the light, enough light may be transmittedthrough the clouds at long wavelengths to clearly image the solar disk.For example, there is a transmission band in the atmosphere from 16 to24 microns, and at 17.8 microns more than 40 percent of light istransmitted through a 1 km horizontal air path at sea level and 46%relative humidity at 15 C. As the imaged disk of the sun translatesacross several pixels in an uncooled focal plane array such as amicrobolometer array, the location and direction of the sun can bedetermined and utilized to define azimuth.

In addition to azimuth sensing, WRI is investigating the accuracyattainable in novel optical self-locating techniques related to the OASdevice. For example, relative positions of near and far objects such assatellites, the moon, planets, and the sun and other stars can beutilized together with an accurate clock in software to extractlocation.

In other embodiments based on man-made celestial objects of the presentteachings for OAS azimuth sensing, self location, and additionalfunctions is described here. These embodiments include a modestconstellation of miniature reflecting satellites (such as reflectingspheres or more complex structures—but passive and insensitive to solarflares and attack) that are placed in near earth, far, or geosynchronousorbits to provide optical reference signals. These orbiting opticalreferences could be spherical or curved reflective sections that areoriented to illuminate the earth below, and could readily exceed theintensities of bright stars. In a more elaborate limit, small passiveretro-reflecting corner-cube reflector satellites can be put in orbitand interrogated optically in embodiments of an optical GPS alternativethat is not jammable. Even relatively simple nanosecond-scale pulsesfrom an advanced OAS and timing circuits can give accuracies on theorder of feet. Active reference satellites containing optical emitters,including infrared, midwave and longwave infrared, and even VLWIRemitters can be used. These emitters include, without limitation,lasers, LEDs, SLEDs, resonators, etc.

Reference is now made to FIG. 6 which includes four time sequentialimages 120, 125, 130, and 135 of Venus taken in broad daylight through afilter passing the 1.3 to 1.7μ spectral IR band, and imager, and anInGaAs Focal Plane Array. This sequence of images 122, 127, 132, and137, are co-registered as Venus moved across the sky.

Reference is now made to FIG. 7 . Using another OAS embodiment, Venuswas photographed in daylight using a filter transmitting the 0.9-1.0micron IR spectral band using a silicon focal plane array. The resultingimage of Venus 150 contains background component 152 and Venus image154.

A detail of this image is shown in image 160 of FIG. 8 and containsbackground 162 and Venus image 164. This photo was taken a bit more thanhalf an hour after sunrise. In the corresponding color image taken inthe visible band at nearly the same time, the image of Venus was notvisible.

Reference is made to FIG. 9 which contains image 170 of the starBetelgeuse photographed during full daylight using the longer wavelengthhalf of the Near Infra-Red (NIR) spectral band. Image 170 containsbackground 172 and Betelgeuse image 174, and is the sum of 256 imagestaken in rapid succession, which lowers the background noise.

Reference is now made to FIG. 10 which includes the sum of 256 broaddaylight images taken in rapid succession 180 of Betelgeuse taken in thenarrower spectral band of roughly 50 nm surrounding one micron. Thelocation of the center of the star image can be interpolated with aprecision of less than a pixel, giving rise to expected azimuthaccuracies of less than 1 mil for common silicon FPAs containing severalmegapixels.

Reference is made to FIG. 11 which includes the sum of 256 broaddaylight images taken in rapid succession 190 of Aldebaran in the samenarrow spectral band around one micron. Cooler stars such as Betelgeuseare relatively brighter in the NIR than those spectrally weighted towardshorter wavelengths.

Reference is now made to FIG. 12 which contains image 200 of the starBetelgeuse taken in broad daylight with an imager of entrance pupildiameter equal to 20 mm. The star image 204 is on background 202. Thisis compared to image 210 of which contains image 214 of Betelgeuse onbackground 212 but this time using the same imager but with entrancepupil diameter of 2.5 mm.

When this same star is imaged through a much smaller lens aperture, theSNR naturally decreases, but it does so gracefully.

Reference is made to FIG. 14 where the SNR is shown for a number ofdecreasing aperture sizes. The signal-to-noise ratio decreases as theentrance pupil diameter of the imager decreases, but in a gracefulfashion. This data shows that OAS operation can be extended to broaddaylight using optimized imagers that are very small.

Various embodiments of OAS sensors use imagers with different fields ofview. Narrow fields of view may allow for detection of dimmer starswhile larger fields of view have a greater chance of including brighterstars in the field. Balancing these trade-offs results in an optimizedwidefield imager, typically ranging from 15° to 120° field of view. Insome embodiments of the OAS extremely wide field of view imagers may beused. Image 230 FIG. 15 shows the night sky through a fisheye imagerwith field of view larger than 180°. This image shows celestial images232, 234, 236, 238, and 240, as well as non-celestial objects such ashouse 242.

There is a large body of art and science to celestial navigation, and ingeneral these techniques can be implemented with great precision and indaylight using the OAS sensor. The OAS can establish the location of thecelestial sphere with high precision and in full daylight and can beused for self location.

Reference is now made to FIG. 16 with illustration 260 that contains theearth 262 (center) surrounded by a Celestial Sphere 264. The celestialsphere is a conceptual surface on which the fixed stars are projected.As the Earth rotates, the celestial sphere exhibits an apparent rotationwith respect to an observer on Earth. The OAS system accurately locatesthe celestial sphere orientation even during daylight, thus providing ahighly accurate azimuth, elevation, and self location sensingcapability. In another embodiment of the OAS, the OAS sensor includes ahighly accurate electronic normal (vertical) vector sensor (such as anaccelerometer) that is used to extract the local earth coordinates fromthe sensed celestial sphere. Figure from Reference [2].

In other embodiments of the present teachings for OAS sensors, selflocation, and other functions are also provided. As discussed earliersome embodiments include a modest constellation of miniature reflectingsatellites (such as reflecting spheres or more complex structures—butpassive and insensitive to solar flares and attack and jamming) that areplaced in near earth, far, or geosynchronous orbits to provide opticalreference signals. This is illustrated by bright points on the celestialsphere shown on celestial sphere 264. These orbiting optical referencescould be spherical or curved reflective sections that are oriented toilluminate the earth below, and could readily exceed the intensities ofbright stars. In a more elaborate limit, small passive retro-reflectingcorner-cube reflector satellites can be put in orbit and interrogatedoptically in embodiments of an optical GPS alternative that is notjammable. Even relatively simple nanosecond-scale pulses from anadvanced OAS and timing circuits can give rise to accuracies on theorder of feet. Other embodiments of such artificial reference satellitesinclude active optical emitters which emit light toward the earth fromtheir specific reference location in orbit. Any optical wavelength canbe used for such emitters which can include without limitation lasers,LEDs, SLEDs, etc. For the case of narrowband emitters narrowband filterscan be used in OAS sensor embodiments to greatly reduce background lightand enhance artificial reference celestial object detectability and OASoperation.

In other embodiments of the present teachings, to an observer on Earth,the celestial sphere rotates during day and night with a period of 24hours. The OAS embodiments equipped with an accurate clock canaccurately determine information such as position from the accuratelylocated celestial sphere orientation that the OAS sensor can determineeven during daylight, thus providing a highly accurate azimuth sensingcapability. The position of the celestial sphere is accuratelycalculable for a given time and location on Earth. Alternatively, if theOAS sensor is used to determine the position of the celestial sphere,and a normal vector pointing from the center of the earth through theobservers location (such as from a vertical sensor like anaccelerometer) is accurately known, then the observers location on Earthis calculable.

In another embodiment of the OAS sensor described above, the sun isimaged at any wavelength and can be used to determine celestial sphereorientation, azimuth, elevation, etc. In such embodiments that operateat increasing wavelengths, the sun's image can be sharply obtainedthrough denser and heavier cloud cover. If the wavelengths used are muchlarger than the diameter of the cloud particles, they will not bescattered as visible light is and the sun should is clearly imageablethrough the clouds. A compact, inexpensive microbolometer focal planearray can be used for extracting the sun's positional data.

References made to FIG. 17 which contains longwave infrared image 290taken with a microbolometer array showing a clouded sky 292 and theimage of the sun's disk 294 visible through the light clouds. The sunimaged in 290 in the Long Wave Infra-Red (LWIR) band using wavelengthsfrom 7-14 microns. In various embodiments of the OAS the position of thesun's disk can be used together with an accurate clock to identify theorientation of the celestial sphere. In an alternative embodiment ofthru-cloud OAS operation, the sun is imaged at long wavelengths onto thefocal plane. Over a short period of time, the disk edge of the sunsweeps across the focal plane and accurately defines the trajectory ofthe sun. This trajectory is used to determine azimuth.

Reference is now made to FIG. 18 . FIG. 18 contains visible band image300 of the sky and longwave infrared band image 310 of the same regionof the sky. The visible image shows a bright sky 302 against dark trees304 while the longwave image 310 shows a dark sky 312 against brighttrees 314. The sky is generally dark in the IR, although the cloud shownis still visible. Different clouds exhibit vastly differing waterdroplet sizes. For clouds with micron sized water droplets, this bandmay be effective for viewing the sun through them. There are longerwavelength spectral windows transmitted by the atmosphere, and they canbe used for OAS operation with larger droplet sized clouds.

Reference is now made to FIG. 19 . FIG. 19 contains visible image 330 ofthe sky 332 with the sun 336 and trees 334. FIG. 9 also containslongwave infrared image 340 of sky 342 and some 346 entries 344. The skyis visibly darker in the longwave band in the disk of the sun is clearlyvisible.

Reference is made to FIG. 20 which illustrates OAS module 400 includinghousing 404 and imager aperture 402. The OAS module 400 contains thespecially optimized imager, focal plane array and electronic processoris described earlier. In this embodiment the OAS module is targeted tooccupy roughly a 1″ to 2″ cube.

Reference is made to FIG. 21 which shows an embodiment of the OAS thatimages the sun's disk through clouds. FIG. 21 includes visible image 420including cloudy sky 422 and bright region 426 obscuring the sun's disk.Visible band Image 430 is a shorter exposure version of image 420 andshows cloudy sky 432 and bright cloudy region 436 obscuring the sun'sdisk. The darker exposure of image 430 reveals that no image of thesun's disk is visible through the clouds in the visible spectral band.Longwave infrared spectral band image 440 taken with a microbolometerfocal plane array shows cloudy sky 442 and clear image of the sun's disk446. This shows operation of OAS embodiments based on imaging the sun'sdisk that are operable through moderate cloud cover. OAS operationthrough heavier cloud cover can be obtained by operating the OAS andeven longer spectral wavelength bands such as in the V LWIR describedearlier. In the images of FIG. 21 regions 424, 434, and 444 are regionsof the images inside the room next to the window through which the skywas photographed.

Reference is now made to FIG. 22 which shows schematically an embodimentof the OAS of the present teachings. Here optical subsystem 510 containsa spectrally optimized imager as described above. Also as describedabove, bandpass filters and edge pass filters can be used to achieve thedesired spectral optimization. The imager 510 is used to image the skyonto focal plane array 520. Due to the inherent spectral sensitivity ofthe focal plane array 520, it also further acts as a spectral filter.Computing or processing element 530 receives the image from the focalplane and produces angular and navigational information such as azimuthfrom the image positions of celestial objects.

As used herein, the singular forms “a,” “an,” and “the” include theplural reference unless the context clearly dictates otherwise. Exceptwhere otherwise indicated, all numbers expressing quantities ofingredients, reaction conditions, and so forth used in the specificationand claims are to be understood as being modified in all instances bythe term “about.”

For the purpose of better describing and defining the present invention,it is noted that terms of degree (e.g., “substantially,” “about,” andthe like) may be used in the specification and/or in the claims. Suchterms of degree are utilized herein to represent the inherent degree ofuncertainty that may be attributed to any quantitative comparison,value, measurement, and/or other representation. The terms of degree mayalso be utilized herein to represent the degree by which a quantitativerepresentation may vary (e.g., ±10%) from a stated reference withoutresulting in a change in the basic function of the subject matter atissue.

Although embodiments of the present teachings have been described indetail, it is to be understood that such embodiments are described forexemplary and illustrative purposes only. Various changes and/ormodifications may be made by those skilled in the relevant art withoutdeparting from the spirit and scope of the present disclosure as definedin the appended claims.

What is claimed is:
 1. A system for determining navigationalinformation, the system comprising: at least one optical referencesatellite, said at least one optical reference satellite providing atleast one optical signal; said at least one optical reference satellitecomprising at least one of optical reflectors or optical emitters; and amodule, said module comprising an optical imager and a focal plane arraydetector; said module receiving said at least one optical signal.
 2. Thesystem of claim 1 wherein said at least one optical reference satellitecomprises at least one optical emitter.
 3. The system of claim 2 whereinsaid at least one optical reference satellite comprises a narrowbandoptical emitter.
 4. The system of claim 1 wherein said at least oneoptical reference satellite comprises at least one optical reflector. 5.The system of claim 1 wherein said at least one optical referencesatellite comprises a reflecting sphere.
 6. The system of claim 1wherein said module further comprises a timing circuit.
 7. The system ofclaim 1 wherein said module further comprises an optical emitter.
 8. Thesystem of claim 1 wherein said module further comprises a narrowbandfilter.
 9. The system of claim 1 further comprises a computing componentreceiving data from the focal plane array detector and outputtingnavigational information.